1. Field of the Invention
The present invention relates to a single-ended input, differential output amplifier (which is also referred to as a balun amplifier).
2. Description of Related Art
In a communication system that utilizes integrated circuits for amplifying the RF input signal, a single-ended input, differential output amplifier (a balun amplifier) is generally located at the most front end of the system for receiving the single-ended input signal from the antenna and converting the single-ended input signal to a differential signal for the following circuit stages that employ differential topology to facilitate the rejection of common mode disturbances in the noisy integrated circuit environment. Since the balun amplifier is located at the most front-end of the radio frequency system, the noise of the balun amplifier itself should be reduced to the minimum.
FIG. 1 is a circuit diagram illustrating a conventional balun amplifier 100. Wherein, the voltage source VS represents a single-ended input signal, and the resistors RL1 and RL2 represent the load impedances. The balun amplifier 100 receives the single-ended input signal VS, amplifies the single-ended input signal VS through the common gate amplifier formed by a n-channel metal oxide semiconductor field effect transistor (which is referred to as an NMOS transistor hereinafter) M1 and the common source amplifier formed by an NMOS transistor M2, and further converts the single-ended input signal VS into a differential output signal for outputting it through output terminals A and B.
The channel noises generated in the NMOS transistors M1 and M2 would also cause noise currents in the balun amplifier 100. The noise currents pass through the resistors RL1 and RL2 and generate the noise voltage of the differential output signal between the output terminals A and B. The design of the balun amplifier 100 can cancel a part of the output noise caused by the channel noises of NMOS transistors M1 and M2.
Regarding the channel noise of the NMOS transistor M1 in the balun amplifier 100, the load currents through the resistors RL1 and RL2 can be the same due to matches among RS, 1/gm1 and 1/gm2, wherein RS is the impedance of the antenna, and gm1 and gm2 are respectively the transconductances of the NMOS transistors M1 and M2. In case that the resistors RL1 and RL2 are the same, the noise voltage between the output terminals A and B can be counteracted in the differential output signal, and the effect of the channel noise generated by the NMOS transistors M1 is nullified.
Regarding the channel noise of the NMOS transistor M2, the cross-coupling of NMOS transistors M3 and M4 increases the impedance Rc looking into the source of NMOS transistor M4, so that an appreciable part of the noise current of the transistor M2 would flow to the ground terminal through the output impedance of the transistor M2. This accordingly reduces the noise current passing through the resistor RL2, i.e., the effect of the channel noise generated by the transistor M2 is also suppressed.